Animals were first cloned from adult cells by Keith Campbell and Ian Wilmut at the Roslin Institute, U.K. This seminal work has been described in U.S. Pat. No. 6,147,276, International Patent Applications WO 97/07669 and WO 97/07668, and in Wilmut et al., Nature 385:810, 1997. The technique involves transferring the nucleus of a cell from the animal to be cloned into a suitable recipient cell. It is thought that the recipient cell causes the genes in the nucleus to be expressed in such a way that a program of embryonic development begins anew. The embryo is then implanted into a surrogate carrier animal for gestation into a viable offspring.
Subsequent to this teaching, other scientists have succeeded in cloning from adult cells. U.S. Pat. No. 5,994,619 reports production of chimeric bovine or porcine animals using cultured inner cell mass cells. U.S. Pat. No. 6,011,197 relates to a method for cloning cows by reprogramming non-embryonic bovine cells using leukocyte inhibitory factor (LIF) and fibroblast growth factor (FGF), then transferring the nucleus into an enucleated oocyte.
International Patent Publication WO 99/21415 reports nuclear transfer for production of transgenic animal embryos. WO 99/05266 and WO 00/52145 propose trans-species nuclear transfer, using bovine oocytes as the recipient cell for a nucleus taken from the donor cell of a different species. WO 99/36510 reports efficient nuclear transfer using fetal fibroblasts. WO 00/25578 proposes a cloning method in which an oocyte is chemically enucleated by exposure to a compound that destabilizes a meiotic spindle apparatus in the recipient cell. WO 00/31237 and WO 99/46982 outline methods for cloning pigs. WO 00/74477 propose a process for animal cloning in which somatic cells denatured by heating are transferred into enucleated metaphase II oocytes. WO 01/00795 describes surgical methods useful for obtaining oocytes from cows for cloning.
Loi et al. (Reprod. Nutr. Dev. 38:615, 1998) discuss embryo transfer and related technologies in sheep reproduction. Wells et al. (Biol. Reprod. 57:385, 1997) report production of cloned lambs from an established embryonic cell line. Liu et al. (Mol. Reprod. Dev. 47:255, 1997) discuss the effect of cell cycle coordination between nucleus and cytoplasm and the use of in vitro matured oocytes in nuclear transfer in sheep embryos. Campbell et al. (Nature 380:65, 1996) report sheep cloned by nuclear transfer from an established cell line.
There is considerable promise in this field for cell therapy and adaptive agriculture. Until the technique of nuclear transfer was developed, genetically modified livestock were made by pronuclear injection (Clark et al., Transgenic Res. 9:263, 2000). Using this methodology, the nucleus of an embryonic cell can be transfected to place a new recombinant gene into the genome of the animal. The new transgene can have any one of a number of desired effects—such as causing secretion of a therapeutic protein into milk, which can then serve as a bioreactor for commercial production (A. J. Clark, Biochem. Soc. Symp. 63:133, 1998, and J. Mammary Gland Biol. Neoplasia 3:337, 1998).
The discovery that animals can be cloned by nuclear transfer from cultured somatic cells provides a new avenue for making animals with a modified genome.